The pulse on the lower channel (blue) is generated whenever the sketch has found 100msec have passed. Following the generation of the pulse the sketch then determines what the next pattern is that needs to be sent out to the 74HC595 (serial to parallel shift register) and clocks it out. The 8 clock pulses that perform the shifting out of the data are showing on the upper channel (yellow). The overall time from when the interrupt occurs and the blue pulse is generated is about 135microseconds. (270 microseconds on one pattern where the shfitout happens twice). This leaves plenty of time for the main program to interact with the user via the LCD Screen and menu buttons.
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